Implement ntopng plugins

Plugins are a convenient way to group together related lua scripts.
Their primary use case is to group user scripts and their alert/status
definition.
The builtin ntopng user scripts and definitions are now
packed into plugins directories. In future, we will support loading of
user created plugins.
Plugins are loaded at startup into some runtime directories and then
used. Other changes provided by this commit include:

- Add sample flow logger plugin
- Initial support for system user scripts
- Rename edge to threshold
- Migrate system probes to user scripts/plugins
- Migrate scripts to more explicit alerts_api.checkThresholdAlert api

scripts/plugins/redis_monitor/user_scripts/system/redis_monitor.lua

@@ -0,0 +1,127 @@
+--
+-- (C) 2019 - ntop.org
+--
+
+local ts_utils = require("ts_utils_core")
+
+local script = {
+  -- This module is enabled by default
+  default_enabled = true,
+
+  -- No default configuration is provided
+  default_value = {},
+
+  -- See below
+  hooks = {},
+}
+
+-- ##############################################
+
+function script.setup()
+  -- This module is always enabled
+  return(true)
+end
+
+-- ##############################################
+
+-- Defines an hook which is executed every minute
+function script.hooks.min(params)
+   if params.ts_enabled then
+      local ifid = getSystemInterfaceId()
+      local stats = script.getStats()
+      local hits_key = "ntopng.cache.redis.stats"
+      local json = require("dkjson")
+      local hits_stats = ntop.getCacheStats()
+
+      local old_hits_stats = ntop.getCache(hits_key)
+
+      if(not isEmptyString(old_hits_stats)) then
+	 old_hits_stats = json.decode(old_hits_stats) or {}
+      else
+	 old_hits_stats = {}
+      end
+
+      ts_utils.append("redis:memory", {ifid = ifid, resident_bytes = stats["memory"]}, when)
+      ts_utils.append("redis:keys", {ifid = ifid, num_keys = stats["dbsize"]}, when)
+
+      for key, val in pairs(hits_stats) do
+	 if(old_hits_stats[key] ~= nil) then
+	    local delta = math.max(val - old_hits_stats[key], 0)
+
+	    -- Dump the delta value as a gauge
+	    ts_utils.append("redis:hits", {ifid = ifid, command = key, num_calls = delta}, when)
+	 end
+      end
+
+      ntop.setCache(hits_key, json.encode(hits_stats))
+   end
+end
+
+-- ##############################################
+
+function script.getRedisStatus()
+   local redis = ntop.getCacheStatus()
+   local redis_info = redis["info"]
+   local res = {}
+
+   for _, k in pairs(redis_info:split("\r\n")) do
+      local k = k:split(":")
+
+      if k then
+	 local v_k = k[1]
+	 local v = tonumber(k[2]) or k[2]
+
+	 res[v_k] = v
+      end
+   end
+
+
+   if redis["dbsize"] then
+      res["dbsize"] = redis["dbsize"]
+   end
+
+   return res
+end
+
+-- ##############################################
+
+local function getHealth(redis_status)
+   local health = "green"
+
+   if redis_status["aof_enabled"] and redis_status["aof_enabled"] ~= 0 then
+      -- If here the use of Redis Append Only File (AOF) is enabled
+      -- so we should check for its errors
+      if redis_stats["aof_last_bgrewrite_status"] ~= "ok" or redis_status["aof_last_write_status"] ~= "ok" then
+	 health = "red"
+      end
+   end
+
+   if redis_status["rdb_last_bgsave_status"] ~= "ok" then
+      health = "red"
+   end
+
+   return health
+end
+
+-- ##############################################
+
+function script.getStats()
+   local redis_status = script.getRedisStatus()
+
+   local res = {
+      -- used_memory_rss: Number of bytes that Redis allocated
+      -- as seen by the operating system (a.k.a resident set size).
+      -- This is the number reported by tools such as top(1) and ps(1)
+      memory = redis_status["used_memory_rss"],
+      -- The number of keys in the database
+      dbsize = redis_status["dbsize"],
+      -- Health
+      health = getHealth(redis_status)
+   }
+
+   return res
+end
+
+-- ##############################################
+
+return(script)